The answer is that if you only have a debit card, you will constantly have to either look online or contact the rental location to find out if they will even accept debit cards, and if so, what the requirements are. It will be a major hassle, and even if a location accepts debit cards today, they may not tomorrow, because all of the rental companies are tightening up their policies.

They are loaning you a $30K plus vehicle. They want to make sure that you are a responsible person who will bring it back as agreed. Whether you agree with them or not, they are increasingly deciding that people who don't have (or can't get) credit cards are not a good risk for them, and they would rather just not rent at all to these folks, than take a chance. It's their vehicle and they can set their policies any way they like.

Short answer: Get a credit card, or deal with the hassle.